1. Field
The following description relates to a method of estimating a frequency offset in an orthogonal frequency division multiplexing (OFDM) system and an OFDM receiver using the same.
2. Description of Related Art
An orthogonal frequency division multiplexing (OFDM) technology is a transmission scheme in which an available frequency band is divided into a plurality of sub-bands, and signals are transmitted by being allocated to orthogonal carrier frequencies of the respective sub bands in an overlapping manner. When compared to a single carrier wave system, the OFDM is widely used as a standardized wireless communication modulation scheme due to its superior performance in a multipath fading channel environment, i.e., a high transmission speed, and superior frequency efficiency.
However, the OFDM system is excessively sensitive to frequency offsets. An integer frequency offset causes interference with respect to shift of indexes of sub-carrier waves of demodulated OFDM symbols modulated through fast Fourier transform (FFT), and a fractional frequency offset destructs the orthogonality of sub-carrier waves, leading to interference between sub-carrier waves (intercarrier interference (ICI)).
In order to prevent significant degradation of performance of the OFDM system due to the frequency offset, various frequency offset estimation schemes have been suggested using multiple symbols. The frequency offset estimation methods for estimating frequency offsets of an OFDM system are largely divided into a training symbol based a frequency offset estimation method and a blind based offset estimation method. The training symbol-based frequency offset estimation method estimates frequency offsets by use of a pilot having a certain structure being transmitted from a transmitter end.
The training symbol-based frequency offset estimation method has slightly poor efficiency in the transmission when compared to the blind-based frequency offset estimation method, but has superior estimation performance. The training symbol-based frequency offset estimation method is shown in a paper published by T. M. Schmidl and D. C. Cox (“Robust frequency and timing synchronization for OFDM”, IEEE Trans. Commun., vol. 45, no. 12, pp. 1613-1621, December 1997). In addition, the frequency offset scheme is also described in a paper published by Keukjoon Bang and others (“A coarse frequency offset estimation in an OFDM system using the concept of the coherence phase bandwidth”, IEEE Transactions on Communications, vol. 49, no. 8, pp. 1320-1324, August 2001).
The open paper “A coarse frequency offset estimation in an OFDM system using the concept of the coherence phase bandwidth” suggests an integer multiple frequency offset estimation that is robust to time offsets by using a coherence phase bandwidth (CPB) in consideration that time offsets affect an integer multiple frequency offset estimation performance in the OFDM system. The method disclosed in the paper provides an integer multiple frequency offset estimation robust to time offsets, but has a computational complexity rapidly increased when the integer frequency offset increases.